Fluorescent proteins

ABSTRACT

The present invention relates to novel variants of the fluorescent protein GFP having improved fluorescence properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/DK96/00051 filed Jan. 31, 1996, the contents of which is fully incorporated herein by reference, andclaims priority of Danish application serial no. 1065/95 filed Sep. 22,1995.

FIELD OF THE INVENTION

The present invention relates to novel variants of the fluorescentprotein GFP having improved fluorescence properties.

BACKGROUND OF THE INVENTION

The discovery that Green Fluorescent Protein (GFP) from the jellyfish A.victoria retains its fluorescent properties when expressed inheterologous cells has provided biological research with a new, uniqueand powerful tool (Chalfie et al (1994). Science 263:802; Prasher (1995)Trends in Genetics 11:320; WO 95/07463).

Furthermore, the discovery of a blue fluorescent variant of GFP (Heim etal. (1994). Proc.Natl.Acad.Sci. 91:12501) has greatly increased thepotential applications of using fluorescent recombinant probes tomonitor cellular events or functions, since the availability of probeshaving different excitation and emission spectra permits simultaneousmonitoring of more than one process.

However, the blue fluorescing variant described by Heim et al, Y66H-GFP,suffers from certain limitations: The blue fluorescence is weak(emission maximum at 448 nm), thus making detection difficult, andnecessitating prolonged excitation of cells expressing Y66H-GFP.Moreover, the prolonged period of excitation is damaging to cellsespecially because the excitation wavelength is in the UV range, 360nm-390 nm.

A very important aspect of using recombinant, fluorescent proteins instudying cellular functions is the non-invasive nature of the assay.This allows detection of cellular events in intact, living cells. Alimitation with current fluorescent proteins is, however, thatrelatively high intensity light sources are needed for visualization.Especially with the blue variant, Y66H-GFP, it is necessary to excitewith intensities that are damaging to most cells. It is worth mentioningthat some cellular events like oscillations in intracellular signallingsystems, e.g. cytosolic free calcium, are very photo sensitive. Afurther consequence of the low light emittance is that only high levelsof expression can be detected. Obtaining such high level expression maystress the transcriptional and/or translational machinery of the cells.

The excitation spectrum of the green fluorescent protein from AequoreaVictoria shows two peaks: A major peak at 396 nm, which is in thepotentially cell damaging UV range, and a lesser peak at 475 nm, whichis in an excitation range that is much less harmful to cells. Heim etal.(1995), Nature, Vol. 373, p. 663-4, discloses a Ser65Thr mutation ofGFP (S65T) having longer wavelengths of excitation and emission, 490 nmand 510 nm, respectively, than the wild-type GFP and wherein thefluorophore formation proceeded about fourfold more rapidly than in thewild-type GFP.

Expression of GFP or its fluorescent variants in living cells provides avaluable tool for studying cellular events and it is well known thatmany cells, including mammalian cells, are incubated at approximately37° C. in order to secure optimal and/or physiologically relevantgrowth. Cell lines originating from different organisms or tissues mayhave different relevant temperatures ranging from about 35° C. forfibroblasts to about 38° C.-39° C. for mouse β-cells. Experience hasshown, however, that the fluorescent signal from cells expressing GFP isweak or absent when said cells are incubated at temperatures above roomtemperature, cf. Webb, C. D. et al., Journal of Bacteriology, October1995, p. 5906-5911. Ogawa H. et al., Proc. Natl. Acad. Sci. USA, Vol.92, pp. 11899-11903, December 1995, and Lim et al. J. Biochem. 118,13-17 (1995). The improved fluorescent variant S65T described by Heim etal. (1995) supra also displays very low fluorescence when incubatedunder normal culture conditions (37° C.), cf. Kaether and Gerdes FEBSLetters 369 (1995) pp. 267-271. Many experiments involving the study ofcell metabolism are dependent on the possibility of incubating the cellsat physiologically relevant temperatures, i.e. temperatures at about 37°C.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide novel fluorescentproteins, such as F64L-GFP, F64L-Y66H-GFP and F64L-S65T-GFP that resultin a cellular fluorescence far exceeding the cellular fluorescence fromcells expressing the parent proteins, i.e. GFP, the blue variantY66H-GFP and the S65T-GFP variant, respectively. This greatly improvesthe usefulness of fluorescent proteins in studying cellular functions inliving cells.

A further purpose of the invention is to provide novel fluorescentproteins that exhibit high fluorescence in cells expressing them whensaid cells are incubated at a temperature of 30° C. or above, preferablyat a temperature of from 32° C. to 39° C., more preferably at atemperature of from 35° C. to 38° C., and most preferably at atemperature of about 37° C.

It is known that fluorescence in wild-type GFP is due to the presence ofa chromophore, which is generated by cyclisation and oxidation of theSYG at position 65-67 in the predicted primary amino acid sequence andpresumably by the same reasoning of the SHG sequence and other GFPanalogues at positions 65-67, cf. Heim et al. (1994). Surprisingly, wehave found that a mutation, preferably a substitution, of the F aminoacid residue at position 1 preceding the S of the SYG or SHG chromophoreor the T of the THG chromophore, in casu position 64 in the predictedprimary amino acid sequence, results in a substantial increase offluorescence intensity apparently without shifting the excitation andemission wavelengths. This increase is remarkable for the blue variantY66H-GFP, which hitherto has not been useful in biological systemsbecause of its weak fluorescence.

The F64L, F64I, F64V, F64A, and F64G substitutions are preferred, theF64L substitution being most preferred, but other mutations, e.g.deletions, insertions, or posttranslational modifications immediatelypreceding the chromophore are also included in the invention, providedthat they result in improved fluorescence properties of the variousfluorescent proteins. It should be noted that extensive deletions mayresult in loss of the fluorescent properties of GFP. It has been shown,that only one residue can be sacrificed from the amino terminus and lessthan 10 or 15 from the carboxyl terminus before fluorescence is lost,cf. Cubitt et al. TIBS Vol. 20 (11), pp. 448-456, November 1995.

Accordingly, one aspect of the present invention relates to afluorescent protein derived from Aequorea Green Fluorescent Protein(GFP) or any functional analogue thereof, wherein the amino acid inposition 1 upstream from the chromophore has been mutated to provide anincrease of fluorescence intensity when the fluorescent protein of theinvention is expressed in cells. Surprisingly, said mutation alsoresults in a significant increase of the intensity of the fluorescentsignal from cells expressing the mutated GFP and incubated at 30° C. orabove 30° C., preferably at about 37° C., compared to the prior art GFPvariants.

There are several advantages of the proteins of the invention,including:

Excitation with low energy light sources. Due to the high degree ofbrightness of F64L-Y66H-GFP and F64L-GFP their emitted light can bedetected even after excitation with low energy light sources. Thereby itis possible to study cellular phenomena, such as oscillations inintracellular signalling systems, that are sensitive to light induceddamage.

As the intensity of the emitted light from the novel blue and greenemitting fluorescent proteins are of the same magnitude, it is possibleto visualize them simultaneously using the same light source.

A real time reporter for gene expression in living cells is nowpossible, since the fluorescence from F64L-Y66H-GFP and F64L-GFP reachesa detectable level much faster than from wild type GFP, and prior knownderivatives thereof. Hence, it is more suitable for real time studies ofgene expression in living cells. Detectable fluorescence may be obtainedfaster due to shorter maturation time of the chromophore, higheremission intensity, or a more stable protein or a combination thereof.

Simultaneous expression of the novel fluorescent proteins under controlof two or more separate promoters.

Expression of more than one gene can be monitored simultaneously withoutany damage to living cells.

Simultaneous expression of the novel proteins using one reporter asinternal reference and the other as variable marker, since regulatedexpression of a gene can be monitored quantitatively by fusion of apromoter to e.g. F64L-GFP (or F64L-Y66HGFP), measuring the fluorescence,and normalizing it to the fluorescence of constitutively expressedF64L-Y66H-GFP (or F64L-GFP). The constitutively expressed F64L-Y66HGFP(or F64L-GFP) works as internal reference.

Use as a protein tag in living and fixed cells. Due to the strongfluorescence the novel proteins are suitable tags for proteins presentat low concentrations. Since no substrate is needed and visualisation ofthe cells do not damage the cells dynamic analysis can be performed.

Use as an organelle tag. More than one organelle can be tagged andvisualised simultaneously in living cells, e.g. the endoplasmicreticulum and the cytoskeleton.

Use as markers in cell or organelle fusions. By labelling two or morecells or organelles with the novel proteins, e.g. F64L-Y66H-GFP andF64L-GFP, respectively, fusions, such as heterokaryon formation, can bemonitored.

Translocation of proteins fused to the novel proteins of the inventioncan be visualised. The translocation of intracellular proteins to aspecific organelle, can be visualised by fusing the protein of interestto one fluorescent protein, e.g. F64L-Y66H-GFP, and labelling theorganelle with another fluorescent protein, e.g. F64L-GFP, which emitslight of a different wavelength. Translocation can then be detected as aspectral shift of the fluorescent proteins in the specific organelle.

Use as a secretion marker. By fusion of the novel proteins to a signalpeptide or a peptide to be secreted, secretion may be followed on-linein living cells. A precondition for that is that the maturation of adetectable number of novel fluorescent protein molecules occurs fasterthan the secretion. This appears not to be the case for the fluorescentproteins GFP or Y66H-GFP of the prior art.

Use as genetic reporter or protein tag in transgenic animals. Due to thestrong fluorescence of the novel proteins, they are suitable as tags forproteins and gene expression, since the signal to noise ratio issignificantly improved over the prior art proteins, such as wild-typeGFP.

Use as a cell or organelle integrity marker. By co-expressing two of thenovel proteins, the one targeted to an organelle and the other expressedin the cytosol, it is possible to calculate the relative leakage of thecytosolic protein and use that as a measure of cell integrety.

Use as a marker for changes in cell morphology. Expression of the novelproteins in cells allows easy detection of changes in cell morphology,e.g. blebbing, caused by cytotoxic agents or apoptosis. Suchmorphological changes are difficult to visualize in intact cells withoutthe use of fluorescent probes.

Use as a transfection marker, and as a marker to be used in combinationwith FACS sorting. Due to the increased brightness of the novel proteinsthe quality of cell detection and sorting can be significantly improved.

Use of the novel proteins as a ratio real-time kinase probe. Bysimultaneous expression of, e.g. F64L-GFP (or F64L-Y66H-GFP), whichemits more light upon phophorylation and a derivative of F64L-Y66H-GFPwhich emits less light upon phophorylation. Thereby, the ratio of thetwo intensities would reveal kinase activity more accurately than onlyone probe.

Use as real-time probe working at near physiological concentrations.Since the novel proteins are significantly brighter than wild type GFPand prior art derivatives at about 37° C. the concentration needed forvisualisation can be lowered. Target sites for enzymes engineered intothe novel proteins, e.g. F64L-Y66H-GFP or F64L-GFP, can therefore bepresent in the cell at low concentrations in living cells. This isimportant for two reasons: 1) The probe must interfere as little aspossible with the intracellular process being studied; 2) thetranslational and transcriptional apparatus should be stressedminimally.

The novel proteins can be used as real time probes based on energytransfer. A probe system based on energy transfer from, e.g.F64L-Y66H-GFP to F64L-GFP.

The novel proteins can be used as reporters to monitor live/dead biomassof organisms, such as fungi. By constitutive expression of F64L-Y66H-GFPor F64L-GFP in fungi the viable biomass will light up.

Transposon vector mutagenesis can be performed using the novel proteinsas markers in transcriptional and translational fusions.

Transposons to be used in microorganisms encoding the novel proteins.The transposons may be constructed for translational and transcriptionalfusions. To be used for screening for promoters.

Transposon vectors encoding the novel proteins, such as F64L-Y66H-GFPand F64L-GFP, can be used for tagging plasmids and chromosomes.

Use of the novel proteins enables the study of transfer of conjugativeplasmids, since more than one parameter can be followed in living cells.The plasmid may be tagged by F64L-Y66H-GFP or F64L-GFP and thechromosome of the donor/recipient by F64L-Y66H-GFP or F64L-GFP.

Use as a reporter for bacterial detection by introducing the novelproteins into the genome of bacteriophages.

By engineering the novel proteins, e.g. F64L-Y66H-GFP or F64L-GFP, intothe genome of a phage a diagnostic tool can be designed. F64L-Y66H-GFPor F64L-GFP will be expressed only upon transfection of the genome intoa living host. The host specificity is defined by the bacteriophage.

Any novel feature or combination of features described herein isconsidered essential to this invention.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 shows a map of pUC 19-GFP plasmid construction;

FIG. 2 a is the DNA (SEQ ID No. 21) and predicted amino acid sequence(SEQ ID No. 22) of GFP;

FIG. 2 b is the nucleotide sequence of GFP (SEQ ID No. 21);

FIG. 3 is the DNA (SEQ ID No. 15) and predicted amino acid sequence (SEQID No. 15) and predicted amino acid sequence of (SEQ ID NO. 16) ofF64L-Y66HGFP;

FIG. 4 is the DNA (SEQ ID NO. 17) and predicted amino acid sequence (SEQID No. 18) of F64L-GFP;

FIG. 5 is the DNA (SEQ ID No. 19) and predicted amino acid sequence (SEQID No. 20) of F64L-S65T-GFP;

FIG. 6 a is a graph of fluorescence emission spectra measured in cellsgrown at 22° C. for 15 hours and excited with light at 398 nm forF64L-GFP, GFP, GFP-N1, F64LS65T-GFP, and lacZ;

FIG. 6 b is a graph of fluorescence emission spectra measured in cellsgrown at 37° C. for 16 hours and excited with light at 398 nm forF64L-GFP, GFP, GFP-N1, F64LS65T-GFP, and lacZ;

FIG. 6 c is a graph of fluorescence emission spectra measured in cellsgrown at 22° C. for 16 hours and excited with light at 470 nm forF64L-GFP, GFP, GFP-N1, F64LS65T-GFP, and lacZ;

FIG. 6 d is a graph of fluorescence emission spectra measured in cellsgrown at 37° C. for 16 hours and excited with light at 470 nm forF64L-GFP, GFP, GFP-N 1, F64LS65T-GFP, and lacZ;

FIG. 6 e is a graph of fluorescence emission spectra measured in cellsgrown at 22° C. for 16 hours and excited with light at 380 nm forF64L-Y66H-GFP, Y66H-GFP and lacZ;

FIG. 6 f is a graph of fluorescence emission spectra measured in cellsgrown at 37° C. for 16 hours and excited with light at 380 nm forF64L-Y66H-GFP, Y66H-GFP and lacZ;

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment of the present invention, the novelfluorescent protein is the F64L mutant of GFP or the blue variantY66H-GFP, said mutant showing increased fluorescence intensity. Apreferred sequence of the gene encoding GFP derived from Aequoreavictoria is disclosed in FIG. 2 herein. FIG. 2 shows the nucleotidesequence of a wild-type GFP (Hind3-EcoR1 fragment) and the amino acidsequence, wherein start codon ATG corresponds to position 8 and stopcodon TAA corresponds to position 722 in the nucleotide sequence. Amicroorganism, E. coli NN049087, carrying the DNA sequence shown in FIG.2 has been deposited for the purpose of patent procedure according tothe Budapest Treaty in Deutsche Sammlung von Mikroorganismen undZellkylturen GmbH, Mascheroderweg 1 b, D-38124 Braunschweig, FederalRepublic of Germany, under the deposition No. DSM 10260. Anothersequence of an isotype of this gene is disclosed by Prasher et al., Gene111, 1992, pp. 229-233 (GenBank Accession No. M62653). Besides, thenovel fluorescent proteins may also be derived from other fluorescentproteins, e.g. the fluorescent protein of the sea pansy Renillareniformis.

Herein the abbreviations used for the amino acids are those stated in J.Biol. Chem. 243 (1968), 3558.

The DNA construct of the invention encoding the novel fluorescentproteins may be prepared synthetically by established standard methods,e.g. the phosphoamidite method described by Beaucage and Caruthers,Tetrahedron Letters 22 (1981), 1859-1869, or the method described byMatthes et al., EMBO Journal 3 (1984), 801-805. According to thephosphoamidite method, oligonucleotides are synthesized, e.g. in anautomatic DNA synthesizer, purified, annealed, ligated and cloned insuitable vectors.

The DNA construct may also be prepared by polymerase chain reaction(PCR) using specific primers, for instance as described in U.S. Pat. No.4,683,202 or Saiki et al., Science 239 (1988), 487-491. A more recentreview of PCR methods may be found in PCR Protocols, 1990, AcademicPress, San Diego, Calif., USA.

The DNA construct of the invention may be inserted into a recombinantvector which may be any vector which may conveniently be subjected torecombinant DNA procedures. The choice of vector will often depend onthe host cell into which it is to be introduced. Thus, the vector may bean autonomously replicating vector, i.e. a vector which exists as anextrachromosomal entity, the replication of which is independent ofchromosomal replication, e.g. a plasmid. Alternatively, the vector maybe one which, when introduced into a host cell, is integrated into thehost cell genome and replicated together with the chromosome(s) intowhich it has been integrated.

The vector is preferably an expression vector in which the DNA sequenceencoding the fluorescent protein of the invention is operably linked toadditional segments required for transcription of the DNA. In general,the expression vector is derived from plasmid or viral DNA, or maycontain elements of both. The term, “operably linked” indicates that thesegments are arranged so that they function in concert for theirintended purposes, e.g. transcription initiates in a promoter andproceeds through the DNA sequence coding for the fluorescent protein ofthe invention.

The promoter may be any DNA sequence which shows transcriptionalactivity in the host cell of choice and may be derived from genesencoding proteins either homologous or heterologous to the host cell,including native Aequorea GFP genes.

Examples of suitable promoters for directing the transcription of theDNA sequence encoding the fluorescent protein of the invention inmammalian cells are the SV40 promoter (Subramani et al., Mol. Cell Biol.1 (1981), 854-864), the MT-1 (metallothionein gene) promoter (Palmiteret al., Science 222 (1983), 809-814) or the adenovirus 2 major latepromoter.

An example of a suitable promoter for use in insect cells is thepolyhedrin promoter (U.S. Pat. No. 4,745,051; Vasuvedan et al., FEBSLett. 311, (1992) 7-11), the P10 promoter (J. M. Vlak et al., J. Gen.Virology 69, 1988, pp. 765-776), the Autographa californica polyhedrosisvirus basic protein promoter (EP 397 485), the baculovirus immediateearly gene 1 promoter (U.S. Pat. No. 5,155,037; U.S. Pat. No.5,162,222), or the baculovirus 39K delayed-early gene promoter (U.S.Pat. No. 5,155,037; U.S. Pat. No. 5,162,222).

Examples of suitable promoters for use in yeast host cells includepromoters from yeast glycolytic genes (Hitzeman et al., J. Biol. Chem.255 (1980), 12073-12080; Alber and Kawasaki, J. Mol. Appl. Gen. 1(1982), 419-434) or alcohol dehydrogenase genes (Young et al., inGenetic Engineering of Microorganisms for Chemicals (Hollaender et al,eds.), Plenum Press, New York, 1982), or the TPI1 (U.S. Pat. No.4,599,311) or ADH2-4c (Russell et al., Nature 304 (1983), 652-654)promoters.

Examples of suitable promoters for use in filamentous fungus host cellsare, for instance, the ADH3 promoter (McKnight et al., The EMBO J. 4(1985), 2093-2099) or the tpiA promoter. Examples of other usefulpromoters are those derived from the gene encoding A. oryzae TAKAamylase, Rhizomucor miehei aspartic proteinase, A. niger neutralα-amylase, A. niger acid stable α-amylase, A. niger or A. awamoriglucoamylase (gluA), Rhizomucor miehei lipase, A. oryzae alkalineprotease, A. oryzae triose phosphate isomerase or A. nidulansacetamidase. Preferred are the TAKA-amylase and gluA promoters.

Examples of suitable promoters for use in bacterial host cells includethe promoter of the Bacillus stearothermophilus maltogenic amylase gene,the Bacillus licheniformis alpha-amylase gene, the Bacillusamyloliquefaciens BAN amylase gene, the Bacillus subtilis alkalineprotease gene, or the Bacillus pumilus xylosidase gene, or by the phageLambda P_(R) or P_(L) promoters or the E. coli lac, trp or tacpromoters.

The DNA sequence encoding the novel fluorescent proteins of theinvention may also, if necessary, be operably connected to a suitableterminator, such as the human growth hormone terminator (Palmiter etal., op. cit.) or (for fungal hosts) the TPI1 (Alber and Kawasaki, op.cit. or ADH3 (McKnight et al., op. cit.) terminators. The vector mayfurther comprise elements such as polyadenylation signals (e.g. fromSV40 or the adenovirus 5 Elb region), transcriptional enhancer sequences(e.g. the SV40 enhancer) and translational enhancer sequences (e.g. theones encoding adenovirus VA RNAs).

The recombinant vector may further comprise a DNA sequence enabling thevector to replicate in the host cell in question. An example of such asequence (when the host cell is a mammalian cell) is the SV40 origin ofreplication.

When the host cell is a yeast cell, suitable sequences enabling thevector to replicate are the yeast plasmid 2μ replication genes REP 1-3and origin of replication.

The vector may also comprise a selectable marker, e.g. a gene theproduct of which complements a defect in the host cell, such as the genecoding for dihydrofolate reductase (DHFR) or the Schizosaccharomycespombe TPI gene (described by P. R. Russell, Gene 40, 1985, pp. 125-130),or one which confers resistance to a drug, e.g. ampicillin, kanamycin,tetracyclin, chloramphenicol, neomycin or hygromycin. For filamentousfungi, selectable markers include amdS, pyrG, argB, niaD, sC.

The procedures used to ligate the DNA sequences coding for thefluorescent protein of the invention, the promoter and optionally theterminator and/or secretory signal sequence, respectively, and to insertthem into suitable vectors containing the information necessary forreplication, are well known to persons skilled in the art (cf., forinstance, Sambrook et al., op. cit.).

The host cell into which the DNA construct or the recombinant vector ofthe invention is introduced may be any cell which is capable ofexpressing the present DNA construct and includes bacteria, yeast, fungiand higher eukaryotic cells.

Examples of bacterial host cells which, on cultivation, are capable ofexpressing the DNA construct of the invention are grampositive bacteria,e.g. strains of Bacillus, such as B. subtilis, B. licheniformis, B.lentus, B. brevis, B. stearothermophilus, B. alkalophilus, B.amyloliquefaciens, B. coagulans, B. circulans, B. lautus, B. megatheriumor B. thuringiensis, or strains of Streptomyces, such as S. lividans orS. murinus, or gramnegative bacteria such as Echerichia coli. Thetransformation of the bacteria may be effected by protoplasttransformation or by using competent cells in a manner known per se (cf.Sambrook et al., supra).

Examples of suitable mammalian cell lines are the HEK293 and the HeLacell lines, primary cells, and the COS (e.g. ATCC CRL 1650), BHK (e.g.ATCC CRL 1632, ATCC CCL 10), CHL (e.g. ATCC CCL39) or CHO (e.g. ATCC CCL61) cell lines. Methods of transfecting mammalian cells and expressingDNA sequences introduced in the cells are described in e.g. Kaufman andSharp, J. Mol. Biol. 159 (1982), 601-621; Southern and Berg, J. Mol.Appl. Genet. 1 (1982), 327-341; Loyter et al., Proc. Natl. Acad. Sci.USA 79 (1982), 422-426; Wigler et al., Cell 14 (1978), 725; Corsaro andPearson, Somatic Cell Genetics 7 (1981), 603, Graham and van der Eb,Virology 52 (1973), 456; and Neumann et al., EMBO J. 1 (1982), 841-845.

Examples of suitable yeast cells include cells of Saccharomyces spp. orSchizosaccharomyces spp., in particular strains of Saccharomycescerevisiae or Saccharomyces kluyveri. Methods for transforming yeastcells with heterologous DNA and producing heterologous polypeptidestherefrom are described, e.g. in U.S. Pat. No. 4,599,311, U.S. Pat. No.4,931,373, U.S. Pat. Nos. 4,870,008, 5,037,743, and U.S. Pat. No.4,845,075, all of which are hereby incorporated by reference.Transformed cells are selected by a phenotype determined by a selectablemarker, commonly drug resistance or the ability to grow in the absenceof a particular nutrient, e.g. leucine. A preferred vector for use inyeast is the POTI vector disclosed in U.S. Pat. No. 4,931,373. The DNAsequence encoding the fluorescent protein of the invention may bepreceded by a signal sequence and optionally a leader sequence, e.g. asdescribed above. Further examples of suitable yeast cells are strains ofKluyveromyces, such as K. lactis, Hansenula, e.g. H. polymorpha, orPichia, e.g. P. pastoris (cf. Gleeson et al., J. Gen. Microbiol. 132,1986, pp. 3459-3465; U.S. Pat. No. 4,882,279).

Examples of other fungal cells are cells of filamentous fungi, e.g.Aspergillus spp., Neurospora spp., Fusarium spp. or Trichoderma spp., inparticular strains of A. oryzae, A. nidulans or A. niger. The use ofAspergillus spp. for the expression of proteins is described in, e.g.,EP 272 277, EP 230 023, EP 184 438.

When a filamentous fungus is used as the host cell, it may betransformed with the DNA construct of the invention, conveniently byintegrating the DNA construct in the host chromosome to obtain arecombinant host cell. This integration is generally considered to be anadvantage as the DNA sequence is more likely to be stably maintained inthe cell. Integration of the DNA constructs into the host chromosome maybe performed according to conventional methods, e.g. by homologous orheterologous recombination.

Transformation of insect cells and production of heterologouspolypeptides therein may be performed as described in U.S. Pat. No.4,745,051; U.S. Pat. No. 4,879,236; U.S. Pat. Nos. 5,155,037; 5,162,222;EP 397,485) all of which are incorporated herein by reference. Theinsect cell line used as the host may suitably be a Lepidoptera cellline, such as Spodoptera frugiperda cells or Trichoplusia ni cells (cf.U.S. Pat. No. 5,077,214). Culture conditions may suitably be asdescribed in, for instance, WO 89/01029 or WO 89/01028, or any of theaforementioned references.

The transformed or transfected host cell described above is thencultured in a suitable nutrient medium under conditions permitting theexpression of the present DNA construct after which the cells may beused in the screening method of the invention. Alternatively, the cellsmay be disrupted after which cell extracts and/or supernatants may beanalysed for fluorescence.

The medium used to culture the cells may be any conventional mediumsuitable for growing the host cells, such as minimal or complex mediacontaining appropriate supplements. Suitable media are available fromcommercial suppliers or may be prepared according to published recipes(e.g. in catalogues of the American Type Culture Collection).

In the method of the invention, the fluorescence of cells transformed ortransfected with the DNA construct of the invention may suitably bemeasured in a spectrometer or a fluorescence microscope where thespectral properties of the cells in liquid culture may be determined asscans of light excitation and emission.

The invention is further illustrated in the following examples withreference to the appended drawings.

EXAMPLE 1

Cloning of cDNA encoding GFP

Briefly, total RNA, isolated from A. Victoria by a standard procedure(Sambrook et al., Molecular Cloning. 2., eds. (1989) (Cold Spring HarborLaboratory Press: Cold Spring Harbor, N.Y.), 7.19-7.22) was convertedinto cDNA by using the AMV reverse transcriptase (Promega, Madison,Wis., USA) as recommended by the manufacturer. The cDNA was then PCRamplified, using PCR primers designed on the basis of a previouslypublished GFP sequence (Prasher et al., Gene 111 (1992), 229-233;GenBank accession No. M62653) together with the UlTma™ polymerase(Perkin Elmer, Foster City, Calif., USA). The sequences of the primerswere: GFP-2: TGGAATAAGCTTTATGAGTAAAGGAGAAGAACTTTT (SEQ ID NO: 1) andGFP-1: AAGAATTCGGATCCCTTTAGTGTCAATTGGAAGTCT (SEQ ID NO:2)

Restriction endonuclease sites inserted in the 5′ (a HindIII site) and3′ (EcoRI and BamHI sites) primers facilitated the cloning of the PCRamplified GFP cDNA into a slightly modified pUC19 vector. The details ofthe construction are as follows: LacZ Shine-Dalgarno AGGA, immediatelyfollowed by the 5′ HindIII site plus an extra T and the GFP ATG codon,giving the following DNA sequence at the lacZ-promoter GFP fusion point:P_(LacZ)-AGGAAAGCTTTATG-GFP. At the 3′ end of the GFP cDNA, the basepair corresponding to nucleotide 770 in the published GFP sequence(GenBank accession No. M62653) was fused to the EcoR1 site of the pUC19multiple cloning site (MCS) through a PCR generated BamHI, EcoRI linkerregion).

The DNA sequence and predicted primary amino acid sequence of GFP isshown below in FIG. 2 a. Another DNA sequence encoding the same aminoacid sequence as shown in FIG. 2 a is shown in FIG. 2 b. To generate theblue fluorescent variant described by Heim et al. (1994), a PCR primerincorporating the Y66H substitution responsible for changing greenfluorescence into blue fluorescence was used as 5′ PCR primer incombination with a GFP specific 3′ primer. The template was the GFPclone described above. The sequence of the 5′ primer is5′-CTACCTGTTCCATGGCCAACGCTTGTCACTACTTTCCTCATGGTGTTCAATGCTTTTCTAGATACCC-3′ (SEQ ID NO:3). Its 5′ end corresponds to position 164 inthe GFP sequence. In addition to the Y66H substitution, the 5′ primerintroduces a A to T change at position 223; this mutation creates a Xba1site without changing an amino acid. The 5′ primer also contains thenaturally occuring Nco1 recognition sequence (position 173 in the GFPsequence). The sequence of the 3′ primer is5′-AAGAATTCGGATCCCTTTAGTGTCAATTGGAAGTCT-3′ (SEQ ID NO:4). Position 3from the 5′ end is the first base of the EcoR1 recognition site thatcorresponds to the 3′ end of the GFP sequence. The resulting PCR productwas digested with Nco1 and EcoR1 and cloned into an Nco1-EcoR1 vectorfragment to reconstitute the entire Y66H25 GFP gene.

E. coli cells carrying an expression vector containing Y66H-GFP weregrown overnight in the presence of 10 micrograms per mlN-methyl-N-nitro-N-nitrosoguanidine. Plasmid DNA was isolated, the 764bp Hind3-EcoR1 insert containing Y66H-GFP was isolated and cloned into aHind3-EcoR1 digested vector fragment, allowing expression of the insertin E. coli. E. coli transformants were inspected for blue fluorescencewhen excited with a 365 nm UV light, and colonies that appeared tofluoresce stronger than wildtype BFP were identified.

10 ng DNA from one particular colony was used as template in a PCRreaction containing 1.5 units of Taq polymerase (Perkin Elmer), 0.1 mMMnCl₂, 0.2 mM each of dGTP, dCTP and dTTP, 0.05 mM dATP, 1.7 mM MgCl₂and the buffer recommended by the manufacturer. The primers used flankthe Y66H-GFP insert. The sequence of the 5′ primer was5′-AATTGGTACCAAGGAGGTAAGCTTTATGAG-3′ (SEQ ID NO:5); it contains a Hind3recognition sequence. The sequence of the 3′ primer was5-CTTTCGTTTTGAATTCGGATCCCTTTAGTG-3′ (SEQ ID NO:6); it contains a EcoR1recognition sequence.

The PCR product was digested with Hind3 and EcoR1 and cloned into aHind3EcoR1 digested vector fragment, allowing expression of the insertin E.coli. E.coli transformants were inspected for blue fluorescencewhen excited with a 365 nm UV light, and colonies that appeared tofluoresce stronger than Y66H-GFP were identified. Plasmid DNA from onestrongly fluorescing colony (called BX12-1A) was isolated and theY66H-GFP insert was subjected to sequence determination. The mutationF64L was identified. This mutation replaces the phenylalanine residuepreceding the SHG tripeptide chromophore sequence of Y66H-GFP withleucine. No other aminoacid changes were present in the Y66H-GFPsequence of BX12-1A. The DNA sequence and predicted primary amino acidsequence of F64L-Y66H-GFP is shown in FIG. 3 below.

EXAMPLE 2

F64L-GFP was constructed as follows: An E.coli expression vectorcontaining Y66H-GFP was digested with restriction enzymes Nco1 and Xba1.The recognition sequence of Nco1 is located at position 173 and therecognition sequence of Xba1 is located at position 221 in theF64L-Y66H-GFP sequence listed below. The large Nco1-Xba1 vector fragmentwas isolated and ligated with a synthetic Nco1-Xba1 DNA linker of thefollowing sequence:

One DNA strand has the sequence:

5′-CATGGCCAACGCTTGTCACTACTCTCTCTTATGGTGTTCAATGCTTTT-3′ (SEQ ID NO:7)

The other DNA strand has the sequence:

5′-CTAGAAAAGCATTGAACACCATAAGAGAGAGTAGTGACAAGCGTTGGC-3′ (SEQ ID NO:8)

Upon annealing, the two strands form a Nco1-Xba1 fragment thatincorporates the sequence of the GFP chromophore SYG with the F64Lsubstitution preceding SYG. The DNA sequence and predicted primary aminoacid sequence of F64L-GFP is shown in FIG. 4 below.

The S65T-GFP mutation was described by Heim et al (Nature vol.373 pp.663-664, 1995). F64L-S65T-GFP was constructed as follows: An E.coliexpression vector containing Y66H-GFP was digested with restrictionenzymes Nco1 and Xba1. The recognition sequence of Nco1 is located atposition 173 and the recognition sequence of Xba1 is located at position221 in the F64L-Y66H-GFP sequence listed below. The large Nco1-Xba1vector fragment was isolated and ligated with a synthetic Nco1-Xba1 DNAlinker of the following sequence:

One DNA strand has the sequence:

5′-CATGGCCAACGCTTGTCACTACTCTCACTTATGGTGTTCAATGCTTTT-3′ (SEQ ID NO:9)

The other DNA strand has the sequence:

5′-CTAGAAAAGCATTGAACACCATAAGTGAGAGTAGTGACAAGCGTTGGC-3′ (SEQ ID NO: 10).

Upon annealing, the two strands form a Nco1-Xba1 fragment thatincorporates the F64L and S65T mutations in the GFP chromophore. The DNAsequence and predicted primary amino acid sequence of F64L-S65T-GFP isshown in FIG. 5 below.

The E. coli expression vector contains an IPTG(isopropyl-thio-galactoside)-inducible promoter. The E. coli strain usedis a del(lacZ)MI5 derivative of K 803 (Sambrook et al. supra).

The GFP allele present in the pGFP-N1 plasmid (available from ClontechLaboratories) was introduced into the IPTG inducible E.coli expressionvector in the following manner:

1 ng pGFP-N1 plasmid DNA was used as template in a standard PCR reactionwhere the 5′ PCR primer had the sequence:

5′-TGGAATAAGCTTTATGAGTAAAGGAGAAGAACTTTT-3′ (SEQ ID NO: 11) and the 3′PCR primer had the sequence:

5′-GAATCGTAGATCTTTATTTGTATAGTTCATCCATG-3′ (SEQ ID NO: 12).

The primers flank the GFP-N 1 insert in the vector pGFP-N1. The 5′primer includes the ATG start codon preceded by a Hind3 cloning site.The 3′ primer includes a TAA stop codon followed by a Bgl2 cloning site.

The PCR product was digested with Hind3 and Bgl2 and cloned into aHind35 BamHIdigested vector fragment behind an IPTG inducible promoter,allowing expression of the insert in E.coli in the presence of IPTG.

The lacZ gene present in the pZeoSV-LacZ plasmid (available fromInvitrogen) was introduced into the IPTG inducible E.coli expressionvector in the following manner:

1 ng pZeoSV-LacZ plasmid DNA was used as template in a standard PCRreaction where the 5′ PCR primer had the sequence:

5′-TGGAATAAGCTTTATGGATCCCGTCGTTTTACAACGTCGT-3′ (SEQ ID NO: 13) and the3′ PCR primer had the sequence:

5′-GCGCGAATTCTTATTATTATTTTTGACACCAGAC-3′ (SEQ ID NO: 14).

The primers flank the lacZ insert in the vector pZeoSV-LacZ. The 5′primer includes the ATG start codon preceded by a Hind3 cloning site.The 3′ primer includes a TAA stop codon followed by an EcoR1 cloningsite.

The PCR product was digested with Hind3 and EcoR1 and cloned into aHind3-EcoR1 digested vector fragment behind an IPTG inducible promoter,allowing expression of the insert in E.coli in the presence of IPTG.

To measure and compare the fluorescence generated in E. coli cellsexpressing GFP, GFP-N1, F64L-GFP, F64L-S65T-GFP, Y66H-GFP, F64L-Y66H-GFPor beta-galactosidase (as background control) under various conditionsthe following experiments were done:

E. coli cells containing an expression plasmid allowing expression ofone of the various gene products upon induction with IPTG were grown inLB medium containing 100 micrograms per milliliter ampicillin and noIPTG. To 1 ml cell suspension was added 0.5 ml 50% glycerol and cellswere frozen and kept frozen at −80 C.

Cells from the −80 C glycerol stocks were inoculated into 2 ml LB mediumcontaining 100 μg/ml ampicillin and grown with aeration at 37 C for 6hours. 2 microliters of this inoculum was transferred to each of twotubes containing 2 ml of LB medium with 100 μg/ml ampicillin and 1 mMIPTG. The two sets of tubes were incubated with aeration at twodifferent temperatures: room temperature (22 C) and 37 C.

After 16 hours 0.2 ml samples were taken of cells expressing GFP,GFP-N1, F64L-GFP, F64L-S65T-GFP, Y66H-GFP, F64L-Y66H-GFP orbeta-galactosidase. Cells were pelleted, the supernatant was removed,cells were resuspended in 2 ml water and transferred to a cuvette.Fluorescence emission spectra were measured in a LS-50 luminometer(Perkin-Elmer) with excitation and emission slits set to 10 nm. Theexcitation wavelengths were set to 398 nm and 470 nm for GFP, GFP-N1,F64L-GFP and F64L-S65T-GFP; 398 nm is near the optimal excitationwavelength for GFP, GFP-N1 and F64L-GFP, and 470 nm is near the optimalexcitation wavelength for F64L-S65T-GFP. For Y66H-GFP and F64L-Y66H-GFPthe excitation wavelength was set to 380 nm, which is near the optimalexcitation wavelength for these derivatives. Beta-galactosidaseexpressing cells were included as background controls. Following themeasurements in the LS-50 luminometer, the optical density at 450 nm wasmeasured for each sample in a spectrophotometer (Lambda UV/VIS,Perkin-Elmer). This is a measure of total cells in the assay.Luminometer data were normalized to the optical density of the sample.

The results of the experiments are shown in FIGS. 6 a-6 f below and canbe summarized as follows:

After 16 hours at 22 C using an excitation wavelength of 398 nm therewere large signals for GFP and F64L-GFP, and detectable signals forGFP-N1 and F64L-S65T-GFP, cf. FIG. 6 a.

After 16 hours at 37 C with an excitation wavelength of 398 nm there wasa large signals for F64L-GFP, a detectable signal for F64L-S65T-GFP, andno detectable signals for GFP and GFP-N1, cf. FIG. 6 b.

After 16 hours at 22 C with an excitation wavelength of 470 nm there wasa large signals for F64L-S65T-GFP, detectable signals for GFP andF64L-GFP, and no detectable signals for GFP-N1, cf. FIG. 6 c.

After 16 hours at 37 C with an excitation wavelength of 470 nm therewere large signals for F64L-S65T-GFP and F64L-GFP, and no detectablesignals for GFP and GFP-N1, cf. FIG. 6 d.

After 16 hours at 22 C with an excitation wavelength of 380 nm therewere detectable signals over background for Y66H-GFP and F64L-Y66H-GFP,cf. FIG. 6 e.

After 16 hours at 37 C with an excitation wavelength of 380 nm there wasno detectable signal over background for Y66H-GFP and a large signal forF64L-Y66HGFP, cf. FIG. 6 f.

To determine whether the differences in fluorescence signals were due todifferences in expression levels, total protein from the E.coli cells(0.5 OD₄₅₀ units) analyzed as described above was fractionated bySDS-polyacrylamide gel electrophoresis (12% Tris-glycine gels fromBIO-RAD Laboratories) followed by Western blot analysis (ECL Westernblotting from Amersham International) with polyclonal GFP antibodies(from rabbit). The result showed that expression levels of GFP, GFP-N1,F64L-GFP, F64L-S65T-GFP, Y66H-GFP and F64L-Y66H-GFP were identical, bothat 22 C and 37 C. The differences in fluorescence signals are thereforenot due to different expression levels.

EXAMPLE 3

Influence of the F64L substitution on GFP and its derivatives whenexpressed in mammalian cells.

F64L-Y66H-GFP, F64L-GFP, and F64L-S65T-GFP were cloned into pcDNA3(Invitrogen, Calif., USA) so that the expression was under control ofthe CMV promoter. Wild-type GFP was expressed from the pGFP-N1 plasmid(Clontech, Calif., USA) in which the CMV promoter controls theexpression. Plasmid DNA to be used for transfection were purified usingJetstar Plasmid kit (Genomed Inc. N.C., USA) and was dissolved indistilled water.

The precipitate used for the transfections were made by mixing thefollowing components: 2 μg DNA in 44 μl of water were mixed with 50 μl2× HBS buffer (280 mM NaCl, 1.5 mM Na₂HPO₄, 12 mM dextrose, 50 mM HEPES)and 6.2 μl 2M CaCl₂. The transfection mix was incubated at roomtemperature for 25 minutes before it was added to the cells. HEK 293cells (ATCC CRL 1573) were grown in 2 cm by 2 cm coverglass chambers(Nunc, Denmark) with approximately 1.5 ml medium (Dulbecco's MEM withglutamax-1, 4500 mg/L glucose, and 10% FCS; Gibco BRL, MD, USA). The DNAwas added to cells at 25-50% confluence. Cells were grown at 37° C. in aCO₂ incubator. Prior to visualisation the medium was removed and 1.5 mlCa²⁺-HEPES buffer (5 mM KCl, 140 mM NaCl, 5.5 mM glucose, 1 mM MgSO₄, 1mM CaCl, 10 mM HEPES) was added to the chamber.

Transfectants were visualised using an Axiovert 135 (Carl Zeiss,Germany) fluorescence microscope. The microscope was equipped with anHBO 100 mercury excitation source and a 40×, Fluar, NA=1.3 objective(Carl Zeiss, Germany). To visualise GFP, F64L-GFP, and F64L-S65T-GFP thefollowing filters were used: excitation 480/40 nm, dichroic 505 nm, andemission 510LP nm (all from Chroma Technologies Corp., Vt., USA). Tovisualise F64L-Y66H-GFP the following filters were used: excitation380/15 nm, dichroic 400 run, and emission 450/65 nm (all from OmegaOptical, Vt., USA).

Cells in several chambers were transfected in parallel, so that, a newchamber could be taken for each sample point. In cases where theincubation extended beyond 8.5 hours the Ca²⁺ precipitate was removed byreplacing the medium.

As shown in Table 1 the F64L mutation enhances the fluorescent signalsignificantly (wild type GFP versus F64L-GFP and F64L-S65T-GFP).Fluorescent cells can be observed as early as 1-2 hourspost-transfection indicating an efficient maturation of the chromophoreat 37° C. Furthermore, the F64L mutation is enhancing other GFPderivatives like the S65T mutant which has a shifted excitation spectrumand the blue derivative which is not detectable in mammalian cellswithout the F64L substitution. (Comment: When comparing the results ofF64L-S65T-GFP and F64L-GFP one has to take into account that theexcitation spectra differ and that the filter set used is optimised forF64L-S65T-GFP. F64L-GFP and WT GFP share the same spectral properties.)

What is claimed is:
 1. A Green Fluorescent Protein (GFP) polypeptidethat has the amino acid sequence of SEQ ID NO:22 with the exception thata Leu residue is substituted for the Phe residue at position 64 of SEQID NO:22 wherein said substituted GFP exhibits increased fluorescence ata temperature of 30° or above, relative to the GFP that has the aminoacid sequence of SEQ ID NO:22, when expressed in a host cell.
 2. Thesubstituted GFP of claim 1, wherein said substituted GFP is produced bysubstituting said Leu residue into the amino acid sequence of SEQ IDNO:22 which is isolated from Aequorea victoria or Renilla reniformis. 3.The substituted GFP of claim 1, wherein the substituted GFP exhibitsincreased fluorescence at about 37° relative to the GFP that has theamino acid sequence of SEQ ID NO:22.
 4. A Green Fluorescent Protein(GFP) polypeptide that has the amino acid sequence of SEQ ID NO:22 withthe exception that a Ile residue is substituted for the Phe residue atposition 64 of SEQ ID NO:22 wherein said substituted GFP exhibitsincreased fluorescence at a temperature of 30° or above, relative to theGFP that has the amino acid sequence of SEQ ID NO:22, when expressed ina host cell.
 5. A Green Fluorescent Protein (GFP) polypeptide that hasthe amino acid sequence of SEQ ID NO:22 with the exception that a Valresidue is substituted for the Phe residue at position 64 of SEQ IDNO:22 wherein said substituted GFP exhibits increased fluorescence at atemperature of 30° or above, relative to the GFP that has the amino acidsequence of SEQ ID NO:22, when expressed in a host cell.
 6. A GreenFluorescent Protein (GFP) polypeptide that has the amino acid sequenceof SEQ ID NO:22 with the exception that a Ala residue is substituted forthe Phe residue at position 64 of SEQ ID NO:22 wherein said substitutedGFP exhibits increased fluorescence at a temperature of 30° or above,relative to the GFP that has the amino acid sequence of SEQ ID NO:22,when expressed in a host cell.
 7. A Green Fluorescent Protein (GFP)polypeptide that has the amino acid sequence of SEQ ID NO:22 with theexception that an amino acid residue selected from the group consistingof Leu, Ile, Val, Gly and Ala is substituted for the Phe residue atposition 64 of SEQ ID NO:22 wherein said substituted GFP exhibitsincreased fluorescence at a temperature of 30° or above, relative to theGFP that has the amino acid sequence of SEQ ID NO:22, when expressed ina host cell.
 8. The substituted Green Fluorescent Protein (GFP)polypeptide of claim 7 wherein a Leu residue is substituted for the Pheresidue at position 64 of SEQ ID NO:22 which is further substituted inthat a His residue is substituted for the Tyr residue at position 66 ofSEQ ID NO:22.
 9. The substituted Green Fluorescent Protein (GFP)polypeptide of claim 7 wherein a Ile residue is substituted for the Pheresidue at position 64 of SEQ ID NO:22 which is further substituted inthat a His residue is substituted for the Tyr residue at position 66 ofSEQ ID NO:22.
 10. The substituted Green Fluorescent Protein (GFP)polypeptide of claim 7 wherein a Ala residue is substituted for the Pheresidue at position 64 of SEQ ID NO:22 which is further substituted inthat a His residue is substituted for the Tyr residue at position 66 ofSEQ ID NO:22.
 11. The substituted Green Fluorescent Protein (GFP)polypeptide of claim 7 wherein a Val residue is substituted for the Pheresidue at position 64 of SEQ ID NO:22 which is further substituted inthat a His residue is substituted for the Tyr residue at position 66 ofSEQ ID NO:22.
 12. The substituted Green Fluorescent Protein (GFP)polypeptide of claim 7 wherein a Gly residue is substituted for the Pheresidue at position 64 of SEQ ID NO:22 which is further substituted inthat a His residue is substituted for the Tyr residue at position 66 ofSEQ ID NO:22.
 13. A Green Fluorescent Protein (GFP) having the aminoacid sequence of SEQ ID NO:
 16. 14. A Green Fluorescent Protein (GFP)having the amino acid sequence of SEQ ID NO:
 18. 15. A Green FluorescentProtein (GFP) having the amino acid sequence of SEQ ID NO:20.